Diamond possesses many desirable physical properties such as hardness, chemical inertness, infrared transparency, and excellent heat conductivity coupled with very high electrical resistivity. Consequently, diamond is a material with many important technological applications such as in optical devices, semiconductors, heat sinks, abrasives, tool coatings, etc. It can also be used as a high-grade, radiation resistant, high-temperature semiconductor. Thus, there is considerable incentive to find more practical ways to synthesize diamond, especially in film form, for these many and varied applications.
Various methods are known for the synthetic production of diamond in film form. One class of methods developed for synthetic diamond deposition is the low-pressure growth of diamond called the chemical vapor deposition (CVD) method. Three predominant CVD techniques have found favor in the literature.
One of these techniques involves the use of a dilute mixture of hydrocarbon gas, typically methane, and hydrogen whose hydrocarbon content usually varies from about 0.1% to 2.5% of the total volumetric flow. The gas is introduced via a quartz tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from about 1750.degree. C. to 2400.degree. C. The gas mixture disassociates at the filament surface and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate, often molybdenum, is heated to a temperature in the region of about 500.degree. C. to 1100.degree. C.
The second technique involves the imposition of a plasma discharge to the foregoing filament process. The plasma discharge serves to increase the nucleation density, growth rate, and it is believed to enhance formation of diamond films as opposed to discrete diamond particles. Three basic plasma systems have been utilized. One is a microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system, and the third is a d.c. plasma system. The RF and microwave plasma systems utilize relatively complex and expensive equipment which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
The third method in use is direct deposit from acetylene as a hydrocarbon-rich oxyacetylene flame. In this technique, conducted at atmospheric pressure, a specific part of the flame is played on a substrate on which diamonds may condense at rates as high as 100 microns/hr. or more. See Y. Matsui et al., Japan Journal of Applied Physics, Vol. 28, p. 178 (1989).
The diamond coatings grow in tension due to growth defects and the "intrinsic strain" therein is proportional to the coating thickness and also the rate of deposition. This intrinsic strain manifests itself by a bowing and/or cracking in the diamond film that has been released from a rigid substrate. The diamond film relieves the tensile stress which was within the diamond coating by bowing into a curved configuration or by cracking.
Substrates of molybdenum have been favored for producing thin diamond films because the CVD diamond tends to nucleate readily on this material. However, removal of a thick diamond film from a molybdenum substrate has posed problems due to strong carbide bonds which form and cause cracking on cool-down and/or require dissolution of the substrate to obtain a self-supporting film. The use of release agents can promote the eventual removal of the film from the substrate; however, once removed, the films still bow in response to this intrinsic strain.
There has been no satisfactory method yet developed to compensate for this bowing problem. Most of the bowing problems have been addressed by growing films with small surface areas so that the bow is not pronounced.
It is desirable to produce CVD diamond films which are flat, which are substantially crack-free, which are easily removed from the substrate upon which they are deposited, and which do not release prematurely during deposition.